Centre for Renewable Energy Sources

Wind Turbine Testing Laboratory EWEC 2007

WT Load Measurements: A Comparison between Load-Based and Analytical Calibration

D. J. Lekou, F. Mouzakis

European Wind Energy Conference 2007, Milan, Italy 2

Load Measurements

Tower (Bending & Torsion) Main Shaft (Bending & Torsion) Blades (Bending) Supporting Systems

Hydraulic pistons Coupling axles Bearing plates

European Wind Energy Conference 2007, Milan, Italy 3

Recommended Sensors

Bending Moments Tower – type T Bending Moments Shaft – type II Torsion – type V Axial Loads – type T

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Load-based Calibration

Apply load Determine influence coefficient

i0iii VVAF

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Analytical Calibration

Electrical calibration Knowledge of material & geometry Definition of influence coefficient

i0iii VVCB

EwF

Load Type B

Bending (tower) T 2(1+ν)

Bending (shaft) II 4

Torsion V 4

Axial T 2(1+ ν)

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Calibration uncertainty

Measured quantity: yNormal (measured value, standard uncertainty)

Type A: analysis of repeated measurementsType B: based on scientific judgment

1N

1j

N

1jkkjxx

kj

N

1j

2x

2

j

2c x,xruu

x

f

x

f2u

x

fyu

kjj

N21 x,...,x,xfy

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Load-based Calibration Uncertainty

LoadMagnitude of loadLoad direction & Load introduction point

SensorsLocation of sensors

Data Acquisition System Other parameters

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Analytical Calibration Uncertainty

Material propertiesE-modulus, Poisson Ratio

Geometrical DataDiameter, Thickness

SensorsLocation, orientation

Data Acquisition SystemGauge factor, Resistor, etc.

Other parameters

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Tension of Plate – Load based

Load Magnitude±0.2% full scale

Load direction±1°

Data Acquisition SystemuV = 2.89mV

AVF

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Tension of Plate - Analytical

CV12

bhEF

Electrical calibration (C)S.g. factorS.g. calibratorType A (linear regression)Data Acquisition System

S.g. misalignment (neglected)

Material PropertiesE-modulusPoisson Ratio

Geometrical dataWidthThickness

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Tension of Plate - Results

Influence Coefficient & uncertainty

Load-based AnalyticalAL (kN/V) uAL/AL (%) AA (kN/V) uAA/AA (%)

11.53 0.34 11.66 3.00

12.09 0.36 11.94 3.00

11.81 0.36 11.69 3.00

12.18 0.31 12.02 3.00

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Tension of Plate - Sensitivity

Influence of material properties on coefficientE-modulusPoisson ratio

NOMINAL Variation E Variation ν

AL*

AA*

E=205, ν=0.29

AA

(E=210)

AA

(E=200)

AA

(ν=0.28)

AA

(ν=0.30)

11.90 12.04 12.34 11.75 12.12 11.9312.34 11.75 12.12 11.93

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Wind Turbine Tower – Load based

Bending momentApplied loadDistance measurementLoad Direction

Indication of s.g. bridge output

Nacelle positionPrevailing wind

AVF

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Wind Turbine Tower - Analytical

Electrical Calibration (C)Material propertiesGeometrical PropertiesData Acquisition system

Strain gauge misalignment (neglected)Strain gauge positioning (neglected)

CV8

hD

12

EF

2

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Wind Turbine Tower - Results

Load calibration2.25%

Analytical calibration3.56%

Target (IEC) uncertainty3%

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Wind Turbine Main Shaft

Similar approach to previousLoad calibration

Blade position

Analytical calibrationOnly E-modulus

CV

D

2

64

dDE

4

1F

44

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Wind Turbine Main Shaft - Results

Load based calibration 1.26%

Analytical calibration 3.15%

Target (IEC) uncertainty3%

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Conclusions - Recommendations

Both methods have limitationsLoad calibration needs high loadsLoad calibration not always feasibleAnalytical calibration results to higher uncertainties

Combination of methods preferred

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Thank you for your attention